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Marie E. Burns, Edward N. Pugh, Jr., Owen P. Gross; Spatiotemporal cGMP Dynamics In Living Mouse Rods. Invest. Ophthalmol. Vis. Sci. 2012;53(14):753.
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© ARVO (1962-2015); The Authors (2016-present)
Single photon responses (SPRs) of rod photoreceptors locally decrease cytoplasmic cGMP, causing closure of cGMP-sensitive channels located in the plasma membrane, and a local decrease in calcium. Whether the fall in cGMP is narrow and deep, or broad and shallow has important consequences for the amplification and fidelity of signaling. The factors that determine the spatiotemporal cGMP profile include the diffusion coefficient for cGMP, the spontaneous rate of cGMP hydrolysis, and the calcium-dependent rate of cGMP synthesis. Our purpose was to experimentally determine the rate of spontaneous cGMP hydrolysis and the effective longitudinal cGMP diffusion coefficient in intact mouse rods, and to subsequently determine the depth and spatial spread of the light-evoked change in cGMP during the SPR.
SPRs of wild type (WT) rods and rods lacking GCAPs-mediated calcium feedback (GCAPs-/-) were measured with suction electrode recordings. "Rogue" SPRs (in which the lifetime of photoactivated rhodopsin, R*, is a prolonged step) provided continuous PDE (G*-E*) activity located to a single disc and a "point sink" of cGMP hydrolysis. A spatiotemporal model of phototransduction was developed, and solved with analytical and numerical methods.
Analysis of the tail phase of SPRs of GCAPs-/- rods with different PDE lifetimes yielded a rate βdark of spontaneous cGMP hydrolysis of 4.1 s-1. Analysis of the amplitude of rogue SPRs in rods with different PDE lifetimes, combined with the value of βdark, yielded an estimate of the effective longitudinal diffusion coefficient DcG in the outer segment of cGMP of 40 µm2 s-1. The spatiotemporal model with the measured values of βdark and DcG,and experimentally measured lifetimes of R* and G*-E*, closely predicted the GCAPs-/- SPRs. With calcium-feedback parameters included, the model also accurately described WT SPRs.
During the WT SPR, the spatiotemporal cGMP profile is both shallow and narrow, never falling more than about 20% of its dark value even at the site of the activated disc. As a consequence, signaling during the rod SPR achieves the maximal amplification allowed by the cGMP-sensitive channels, and preservation of signal linearity within individual signaling domains of ~ 2 µm.
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